Most integrated circuit (IC) devices are packaged so as to locate the input/output (I/O) connections along the outer edges or perimeter of the device. These devices have metal leads which are mated to a printed wiring board (PWB) by insertion into plated through holes (PTH) or by placement of the leads on top of metal pads on the PWB surface. Electrical connection is made during subsequent soldering operations. Another common type of device has the I/O connections, in the form of pins, arrayed across the area of the package body instead of along the perimeter of the package. This type of package is generally known as a pin grid array (PGA). These metal pins are most commonly inserted into a grid of PTHs formed in the PWB. Less commonly, the ends of the PGA pins are placed in contact with metal pads arrayed across the surface of the PWB. Electrical connection is made during subsequent soldering operations. The method of electrically connecting device I/O leads or pins to solder pads on the surface of PWBs is referred to as surface mount technology (SMT), while the method of connecting using leads or pins inserted into PTHs is referred to a through hole assembly (THA).
The PGA has the advantage of achieving high I/O numbers, typically greater than 100, in a relatively small area while maintaining a fairly coarse I/O pin pitch of 0.100 inch. In contrast, a perimeter I/O device must have leads on much tighter pitches, typically less than 0.050 inch, in order to achieve an equivalent number of I/O interconnections as the PGA. The fine pitch used for a high I/O perimeter device is commonly 0.4 mm to 0.5 mm. It is difficult to achieve high yields and productivity with such fine pitch perimeter soldering.
However, while the PGA has advantages over perimeter I/O devices, one problem with the PGA package is the ease with which the I/O pins can be bent or damaged. Bent pins typically require a pin straightening step before the package can be used. However, more often than not, the pins are not repairable once bent, and thus an electrically functional IC must be scrapped.
Another type of device called a ball grid array (BGA) provides advantages in the areas of package size and the system PWB cost. BGA packages have I/O connections composed of solder balls which are joined to the package prior to PWB assembly. The solder balls are more resistant to mechanical damage during normal handling than either I/O leads or pins because the solder balls cannot be bent. They can, however, be flattened. If high melting temperature solder balls, such as 90% lead /10% tin solder, are used in the BGA, these solder balls do not melt during PWB solder reflow so as to maintain a relatively high standoff. A solder ball which has suffered some mechanical damage or flattening from handling, however, may not make electrical contact to the system board during the joining operation, thus resulting in an open circuit. Another problem facing the BGA package is the relatively low thermomechanical stress compliance afforded by the small diameter balls. The BGA I/O ball pitch of 1 mm to 1.5 mm restricts the ball diameter to an approximate range of 0.4 mm 1.0 mm since larger solder balls could result in short circuits between the balls. The separation distance between the BGA substrate and the PWB can be from 0.3 mm to 1.0 mm for these ball sizes. The short length of these solder joints are subjected to the thermal stresses which result from thermal expansion differences in the BGA substrate and the PWB during the device's operation. While it is more desirable to have higher solder joint standoffs to accommodate large differences in thermal expansion, the aforementioned limitations on ball sizes and pitches prevent such higher solder joint standoffs.
Another prior art device uses high temperature solder columns in place of solder balls to provide a higher standoff than is possible with solder balls having the same diameter as the solder columns. However, despite the advantages over solder balls, these solder columns still exhibit fatigue with temperature cycling of the devices due to the thermally induced stresses, and therefore eventually break.